You will need:
A Kinder Egg
Some string (optional)
Dremel multi tool
Hot glue glue (optional)
And most important, KITTY TREATS!
Step 1: 1st, Eat the Chocolate and Assemble the Toy.
You'll be left with a yellow plastic egg, draw an outline of the size hole you want. Make it just a bit bigger than the kitty treats you want to use.
Step 2: DO NOT Use a Knife With This Step, the Plastic Can Split.
Use your Dremel multi-tool to cut the outline out, and drill a hole for the string in the end with the hole. This makes it easy for kitty to carry without the treats falling out.
Step 3: Add the String
Thread the string through the small hole, tie a knot . Add a small blob of hot glue to the knot and pull the string, this will attach the string to the inside of the egg.